The present invention relates to a timekeeping signal source for an electronic timepiece.
Electronic timepieces utilizing quartz crystal controlled oscillator circuits as a standard frequency signal source are now in widespread use. Such timepieces can be broadly divided into two basic types, i.e. wristwatches and clocks. A significant difference between these two types lies in the fact that the power consumption of an electronic wristwatch must be made as small as possible, in order to enable the battery lifetime to be maximized. This is necessary due to the extremely small size of battery used in such a timepiece, which has a correspondingly low power supply capacity. In the case of an electronic clock, the power consumption limitations are much less stringent. It is for this reason that the quartz crystal oscillator circuit of an electronic clock generally operates at a relatively high frequency, of the order of 4,194,304 Hz, while in the case of an electronic wristwatch, the quartz crystal oscillator generally operates at a much lower frequency, e.g. 32,768 Hz. The reason for this difference in frequencies is that the power consumption of the frequency divider circuit of the timepiece, which produces a timekeeping signal of, say one second period, from the crystal oscillator circuit output signal, increases significantly as the oscillator circuit frequency is increased. In addition, the power consumption of a quartz crystal oscillator circuit operating at a frequency of the order of 30,000 Hz is appreciably lower than that of an oscillator circuit operating in the region of 4 megaherz. However, use of a high crystal oscillator frequency has the advantage of providing a much higher degree of frequency stability, with respect to ambient operating temperature variations and long-term frequency drift, as compared with a lower oscillation frequency such as is utilized in electronic wristwatches.
Another problems which arises with regard to using a relatively low frequency quartz crystal oscillator circuit in an electronic timepiece lies in the area of frequency adjustment. It is necessary to adjust the timekeeping rate of an electronic timepiece extremely precisely, to provide accurate timekeeping, this adjustment being generally carried out at the time of manufacture. The generally utilized method of adjustment hitherto, for electronic wristwatches, has been to couple a variable capacitor to the quartz crystal oscillator circuit of the timepiece, and adjust this capacitor to vary the frequency of oscillation very slightly, in order to adjust the timekeeping rate of the timepiece. Such a method has the serious disadvantage that the capacitance value of such a variable capacitor will vary, over the long term, thereby resulting in drift of the timekeeping rate from the initially adjusted value. A preferable method of timekeeping rate adjustment is to perform digital adjustment, for example by varying the division ratio of the frequency divider circuit of the timepiece. However, with the low frequency of oscillation of the oscillator circuit generally used in an electronic wristwatch, such a digital adjustment method provides only a coarse degree of adjustment resolution, so that the timekeeping rate cannot be precisely set to a desired value. For this reason, digital adjustment of the timekeeping rate of an electronic wristwatch has not been utilized hitherto, on a practical basis, in spite of the advantages of such a method.
A timekeeping signal source according to the present invention for an electronic timepiece, and in particular for an electronic wristwatch, provides the advantages described above which are attained by utilizing a high frequency quartz crystal oscillator circuit, namely, a high degree of frequency stability against ambient operating temperature variations together with a low level of long-term frequency drift, and also enables the timekeeping rate to be precisely adjusted in a digital manner, with the adjustment resolution being within to one period of a high frequency reference signal produced by a quartz crystal high frequency oscillator circuit. At the same time, since the timekeeping signals are produced by frequency division of a relatively low frequency signal, which is obtained by controlling the frequency of oscillation of a relatively low frequency quartz crystal oscillator circuit, in accordance with the high frequency reference signal, the advantage of low power consumption is also provided by a timekeeping signal source according to the present invention. Thus, a timekeeping signal source according to the present invention enables an electronic timepiece to be produced having a highly accurate and stable timekeeping rate, yet also having a low level of power consumption and hence a long battery lifetime.